Tuesday, 3 February 2015
Often I am asked how to take photos of the Northern Lights. How does one need to set up a camera to be successful? The answer to this question implies the suitability of a specific camera to the job. There are, of course, other considerations as well, but here I want to concentrate on the three main parameters that determine how sensitive the camera is to light, i.e. ISO sensitivity, exposure time, and aperture.
For nighttime photography of the sky, where focus is at infinity, the aperture should be set to the widest possible value. Apertures are given as ratios like f/2.8. The aperture is largest, when the so-called f-number is smallest. For instance, an aperture of f/2.0 will let twice as much light onto the camera sensor as an aperture of f/2.8.
Once the aperture is set wide open, one has to balance between exposure time and ISO value. ISO defines the sensitivity of the sensor, but increasing sensitivity (higher ISO values) implies a grainier or noisier image. Thus one should keep this value low if possible.
But lowering the ISO value implies increasing exposure time to catch the same amount of light. For instance, an exposure time of 4 seconds at ISO 400 is the same as an exposure time of 2 seconds at ISO 800.
However, one cannot, for two reasons, increase exposure time arbitrarily. First, if the Northern Lights are really active and fast moving, one needs to keep the exposure time as low as possible in order to freeze the motion as well as possible. Otherwise the image is just a large blurry blob of green light.
Secondly, at some point the rotation of the Earth will cause the stars in the image to appear as lines rather than dots. A commonly used rule states that one should keep the exposure time below a value of 500 divided by focal length (in seconds). For example a lens with a focal length of 50 mm will allow for a maximum exposure of 500/50 = 10 seconds. Note that, if you use a crop sensor, you have to multiply the focal length by 1.5 or 1.6 depending on your camera.
The diagram above (click it for a larger version) will help with this balancing act. The dashed horizontal lines at the top give the maximum exposure time according to the above rule for a number of popular focal lengths. Check where your lens reaches its limit here at its most zoomed out setting.
Next, choose the diagonal line with the f-number of your widest possible aperture. Then you can move along this line to see which combination of ISO values and exposure times to start with when photographing the Northern Lights.
For example, if your lens is a 24mm f/1.4 on a full-frame camera, then you can go to a maximum exposure time of 20 seconds, and an exposure time of 4 seconds at ISO 400 or 2 seconds at ISO 800 would work fine.
The photo above was taken at only 0.5 seconds exposure time at ISO 800 using a 24mm f/1.4 lens. This shows that the graph is not a definite rule. The graph provides a starting point, meaning if you set your camera using these values, you will succeed taking photos of the Northern Lights. But if the lights are bright, you might get away with shorter exposure times or lower ISO values, thus you can freeze faster motion or get less sensor noise.
Finally, this chart will tell you also if your equipment is enough for the job. Just check where your setup would be on this chart. Furthermore, modern camera sensors get more sensitive all the time, and some cameras nowadays get decent results with ever higher ISO values. Thus I encourage you to give it a try and see how far you can push your camera.
Photo and graph: Thomas Ulich.